The fan of a turbofan gas turbine is required to generate thrust by increasing the pressure of the air-flow through the fan, or bypass, duct and in addition the fan may be used to supercharge the core engine of the gas turbine engine. Stator vanes are required in the airflow through the fan duct and in the airflow to the core engine to remove the swirl of the air from and to increase the static pressure of the air by extracting kinetic energy from the airflow.
As the pressure ratio increases to provide more thrust from the airflow through the fan duct or to provide more supercharging of the core engine, more aerodynamic loading is imposed on the stator vanes, especially if the fan speed is limited to increase rotor efficiency. As the aerodynamic loading increases it is more difficult for the airflow to sustain the pressure rise without separation of the airflow from the air washed surfaces of the components, especially near the end walls between the stator vanes, where the annulus boundary layer flow over the end walls interacts with the vane boundary layer flow over the vanes.
It is known to control the separation of the boundary layer on a surface by using a pump to suck the boundary layer from the surface. This arrangement requires the use of a pump. It is also known to remove the boundary layer from the compressor at a position and to return the boundary layer to the compressor upstream of the position. This arrangement reintroduces high temperature air into the compressor and inherently requires more shaft work to achieve a given pressure ratio than if the low temperature air were removed from the compressor.